1. Field of the Invention
The present invention relates to a system and a method for calculating a voltage stability risk index in power system and, more particularly, to a system and a method for promptly monitoring transient voltage stability in real time using voltages measured.
2. Background of the Related Art
In operating a power system, an electric power company controls voltage through two methods. First, the voltage, which is measured and transferred from each main bus, is shown on a display board and the electric power company passively controls the voltage so that the displayed voltage is maintained at a value ranging from 95% to 105% of a rated voltage. Such a method is very simple and effective in controlling voltage for a normal condition, but not helpful for a condition requiring urgent control, for example, when a fault or contingency in the system has occurred due to a disturbance. Moreover, this method cannot provide the information about the sensitivity of a bus voltage.
Second method is to control voltage by analyzing bus voltage sensitivity and calculating a voltage collapse point at a normal state using a power flow calculation method. To apply the power flow calculation, all the data related to a power system have to be inputted: a) impedance data of a power transmission network can be prepared in advance, or may be changed according to the topology of the power transmission network, b) the data on electric power amount and load amount, on the other hand, have to be directly measured and transferred so as to reflect the continuously changing condition of a system. However, it is impossible to measure the electric power amount and load amount for the whole buses in a power system comprising hundreds or thousands of buses. Thus, the voltage, the electric power amount, and the load amount on only some main buses are measured and the data on the rest buses unmeasured are estimated based on the measured electric power amount and load amount. The data comprising the data already prepared, the data measured, and the data estimated, are inputted into a power flow calculation program. The calculation period is more than ten minutes.
One of the voltage stability analyses based on a power flow calculation is a sensitivity analysis on a bus voltage, which calculates a sensitivity representing a minute change of voltage (V) to a minute change of reactive power (Q). If the V−Q sensitivity is positive, it means that voltage stability is stable. The smaller the V−Q sensitivity is, the more robust the bus voltage is. If the voltage stability is reduced, the V−Q sensitivity increases and has an infinite value at a voltage collapse point. Negative V−Q sensitivity means unstable in terms of the voltage stability. Another voltage stability analysis is to calculate a voltage collapse point. The voltage collapse point is a maximum power transmission point at which a power system transmission network can normally transfer, and a point at which the voltage is collapsed. The difference between a voltage collapse point measured and a system operation point may be used as a voltage stability margin. The voltage collapse point is simulated by assuming a scenario because the data on load amounts and electric power amounts cannot be obtained in advance.
However, voltage control methods using such conventional voltage stability analyses cannot completely monitor the voltage in an electric power system and fully reflect voltage characteristics of the whole power system because they uses partial data measured only for main buses. In addition, voltage control methods using such conventional voltage stability analyses cannot achieve a real time voltage monitoring in a power system and cannot represent the soundness of bus voltage at a transient condition.